Strength training improves communication between your brain and muscle fibers, causing great improvements in strength and exercise performance. In fact, untrained individuals can only activate a portion of their existing muscle mass at any given time. Understanding how resistance training affects neuromuscular, or brain-to-muscle, communication enables informed exercise decisions.
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Rate of Activation
A motor unit contains one motor neuron -- a cell located in your central nervous system that transmits information by electrical and chemical signaling -- and all muscle fibers connected to it. When an electrical signal from your brain stimulates a given motor neuron, all fibers associated with that neuron contract, or activate. For example, motor neurons for eye muscles may control 10 to 100 fibers, while motor neurons for large leg muscles may control thousands of muscle fibers. Weightlifting increases the rate of motor-unit recruitment and enables greater strength during activity.
Like many physiological systems, motor units can become tired during exercise. As your neuromuscular system fatigues, ability for motor unit recruitment subsides and you may experience reduced strength. However, strength training increases the length of time that a motor unit can stay activated without becoming tired or overworked. Therefore, neuromuscular adaptations enhance muscular endurance, or your ability for muscle action over time. For example, enhanced motor-unit endurance enables you to perform a greater amount of pushups or pullups in succession.
Physical activity requires activation of multiple muscle groups using numerous motor units. If sedentary, your brain may activate required motor units at slightly different times -- causing inefficient movement. On the other hand, strength training aids synchronization of motor unit recruitment, or your ability to activate needed motor units at the exact time for maximal strength production and movement efficiency. Synchronized motor unit activation works much like rowers rowing a boat in sync versus the rowers rowing at different times.
Recruitment of Fast-Twitch Fibers
Human muscles contain fast-twitch and slow-twitch muscle fibers. Slow-twitch fibers perform low-intensity, endurance-based actions, such as walking, while fast-twitch fibers support high-intensity, power-based activities, such as sprinting or lifting heavy weights quickly. Generally, greater activation of fast-twitch fibers increases your speed and strength during physical activity. Over time, resistance training develops communication between your brain and fast-twitch motor units, enabling greater activation of those fibers. Neuromuscular adaptations can occur within weeks of a new strength-training program.
Breaking Through Plateaus
After your body has made the necessary neuromuscular adaptations, your current strength-training program will become less challenging. At this point, you will hit a training plateau if you continue to use the same program. A plateau in strength training means that you no longer see improvements by applying the same stimulus. The overload principle states that to continue getting stronger, you will need to increase the intensity of your strength-training program. Try adding 5 or 10 more pounds to each lift in your program. Work back up to being able to perform the same amount of reps and sets you could with the lighter loads used in your original program.